Two Case Reports of Subacute Thyroiditis after Receiving Vaccine for COVID-19

The ongoing COVID-19 pandemic, caused by a coronavirus named SARS-CoV-2, has struck the planet with great force. As of December 2019, the virus has made its devasting route across all continents . In January 2022, the World Health Organization (WHO) registered over 5.5 million COVID-19 related deaths. Most of these people had suffered from pneumonia and acute respiratory distress syndrome , and in some cases, extensive damage to all organ systems. To get hold of this pandemic, it was vital to find effective vaccines against it. The two vaccine candidates BNT162b2 (BioNTech/Pfizer) and ChAdOx1 (University of Oxford and AstraZeneca) offer a high level of protection against COVID-19 by providing immunity due to antibody production against the spike protein of SARS-CoV-2. In addition to general side effects, immunological side effects such as subacute thyroiditis can follow the vaccination. This transient inflammatory condition of the thyroid gland is characterized with hyperthyroxinemia, inflammation, pain, and tenderness in the thyroid region, as well as an elevation of serum thyroglobulin concentration. There are only a few reports on the occurrence of this disease after receiving a COVID-19 vaccine. We present two cases of subacute thyroiditis after vaccination with the vaccines BNT162b2 and ChAdOx1 and try to enlighten the problem of immunological phenomena after vaccination. It must be discussed whether cross-reactivity of the spike protein and tissue proteins such as thyroid peroxidase (TPO), an “autoimmune/inflammatory syndrome by adjuvants” (ASIA), or the circulating spike protein itself after vaccination are responsible for the SAT.


Background
SAT, is an inflammatory disease of the thyroid gland , which primarily affects women and is manifested by pain and tenderness. Furthermore, patients mention fever, general malaise, myalgia, and arthralgia [10]. ese typical clinical symptoms and the presence of elevated free thyroxine (fT4) and free triiodothyronine (fT3) with suppressed thyroid-stimulating hormone (TSH) as well as increased erythrocyte sedimentation rate (ESR) lead to the diagnosis of SAT. Symptoms of thyroid hormone excess may occur in the initial phase, and hypothyroidism affects about 5-10% of patients at annual follow-up [31].
e thyroid gland appears mostly focally hypoechogenic and of normal or enlarged size with no hypervascularity on ultrasound [32]. In addition, 99mTc-pertechnetate uptake is markedly reduced, indicating an inadequate metabolic pathway during inflammation [33].
To date, there is no known definitive cause for this disease.
ere are some studies that show an association between SAT and HLA-B35 in several ethnic groups, as well as a family-related occurrence [34]. Previous viral infections, such as influenza, coxsackie virus, and mumps virus, have been discussed as potential causes for a long time, but so far this has not been proven with absolute certainty [35]. is theory is partly supported by observing the seasonal occurrence of the SAT from summer to early autumn in some studies [36,37]. e search for a viral infection is often useless unless there is a therapeutic consequence for the viral disease as such.
Recently, there have been several case reports that showed the occurrence of SAT after infection with SARS-

Case Presentation I
A 36-year old woman presented to our endocrinology outpatient clinic in April 2021, four weeks after receiving her second vaccine dose against COVID-19 with BNT162b2 (BioNTech/Pfizer, Comirnaty ® ). She reported that she had developed neck pain one day after receiving this second dose.
Following symptom onset, she had presented to an ear, nose, and throat doctor. After a laryngoscopy, which found no pathological findings, the doctor performed an ultrasound examination of the thyroid gland, which was assessed to be abnormal. In addition, there was an increased C-reactive protein (CRP), leukocytosis, and an increased thyroglobulin in the external blood tests, so the colleague began a three-day therapy with prednisolone 60 mg/d to treat his suspected diagnosis of SAT. A few hours after starting this treatment the pain disappeared but returned shortly after prednisolone was discontinued. e symptoms worsened over the following weeks, so the woman presented herself in our endocrinology outpatient clinic for further examination and treatment advice. At that time, the patient complained about neck pain, restlessness, as well as fatigue.
She did not have any known preexisting diseases, especially no previous history or family history of thyroid disease or COVID-19. e patient was a cigarette smoker.
2.1. Investigation. At examination, she was 170 cm tall and weighed approximately 55 kilograms (body mass index, 19 kg/m 2 ). ere were no pathological findings regarding heart rate, blood pressure, and temperature. e neck pain could be induced by palpating the thyroid gland. Our initial biochemical blood tests showed a slight leukocytosis as well as moderately increased CRP and significantly increased ESR. In addition, there was a markedly elevated thyroglobulin level with normal fT4, fT3, and TSH indicating an euthyroid state, most likely due to the fact that the patient presented four weeks after the onset of symptoms. No thyroid peroxidase autoantibodies (TPO-Ab), TSH receptor stimulating antibodies (TRAb), or antithyroglobulin antibodies (TgAb) were detected (Table 1).
Ultrasound examination of the thyroid gland showed a hypoechogenic texture with pseudonodular lesions (Figure 1). e volume was about 18 ml, and some enlarged nonsuspicious lymph nodes were seen. e appearance was typical for SAT, and the pain could be induced by pressure with the ultrasound probe right above the thyroid gland.
ere was no evidence of a current corona infection in the rapid antigen test.

Treatment.
Based on strong evidence suggesting SAT with normal thyroidal function at presentation, we discussed the treatment options with our patient. We offered her therapy with nonsteroidal anti-inflammatory drugs (NSAIDs) or a longer course of prednisolone therapy. She decided on the former, and we advised her to begin a prednisolone therapy if there was no improvement on NSAID with a starting dose of 30 mg/d with tapering the daily dosage by 5 mg every 5-7 days.

Outcome and Follow-Up.
During a telephone call two weeks later, the patient reported that the neck pain had worsened while she was taking ibuprofen and that she had developed subfebrile temperatures. erefore, as recommended, she started prednisolone therapy, which promptly resulted in improvement. She took prednisolone in a tapering scheme over six weeks. e symptoms did not return after stopping the prednisolone, and she has been fine. An intermittent unintentional weight loss of five kilograms was also resolved. e ultrasound examination showed an almost complete recovery. yroglobulin decreased by about 75%, and the CRP was normal. Mild leukocytosis may have been explained by smoking. Her thyroid function tests still showed an euthyroid state. e patient also complained of pain in the thyroid region and hoarseness; in secondary findings, a decreased TSH was detected. erefore, a referral to our endocrinology outpatient clinic was arranged. At the time of the appointment, the local symptoms were decreasing and the patient was fine. ere was no history of thyroid disease, neither in the patient nor in their family.

Investigation.
e man was in good condition. Blood pressure was normal, and the heart rate was moderately elevated (100 bpm). Due to decreased appetite, he had lost eight kilograms. e thyroid gland was slightly enlarged and palpable without tenderness. Blood examination revealed moderately elevated CRP, normal white blood cell count, mild normochromic normocytic anemia, and elevated ESR. In addition, there was a reduced TSH with normal fT3 and fT4 and increased TgAb, which we assessed as unspecific (Table 1). Other antibodies related to the thyroid gland were negative (TRAb and TPO-Ab).
Ultrasound examination of the thyroid showed a hypoechogenic texture and a diffusely hypoperfused parenchyma. e volume was slightly enlarged (36 ml) and there were some enlarged nonsuspicious lymph nodes (Figure 2). A COVID-19 rapid antigen test was negative.

Treatment.
No special treatment was required because of the regressive symptoms.

Outcome and Follow-Up.
At the recommended appointment three months later, the patient felt well. Blood tests showed mild anemia but no more signs of inflammation, and the TSH was normalized. Sonographic examination of the thyroid gland showed a reduced volume. ere was no tenderness or pressure on the thyroid region.

Discussion
e current COVID-19 pandemic is causing various medical problems. In addition to the main infections, e.g., lung infection, the infection affects the entire body system. Initial studies by Somasundaram et al. in June 2020 suggested possible damage to the endocrine system by SARS-CoV-2 [39]. Only a few months later, the first reports underscored this suspicion [40]. e thyroid gland could be affected by several mechanisms.
First of all, the virus enters human tissues using the angiotensin-converting enzyme 2 (ACE2) as a cell receptor, which is strongly expressed in thyroid cells and can probably damage the thyroid gland by direct viral cytopathic effects [41,42]. Early research had shown that there are some homologies between amino acid sequences of, e.g., the spike glycoprotein subunits S1 and S2 from the virus and tissue proteins such as thyroid peroxidase (TPO). is can cause autoimmunity against thyroid cells or exacerbate an existing autoimmunity [43].
In addition, uncontrolled immune responses to the virus in severe cases of COVID-19 should be discussed. is leads to an extensive inflammatory state and an enhanced 1/ 17 immune response, which results in the release of inflammatory cytokines that are also seen in autoimmune thyroid diseases [44].
Furthermore, the exact pathomechanism of SAT in general is unknown. It is hypothesized that autoreactive T-cells generated by molecular mimicry on thyroid cells promote an autoimmune response against the thyroid gland. Another explanation is that a virus infection results in the presentation of viral products or virus antigens by host cells. As a result, the thyroid cells are recognized as foreign and destroyed by the immune system [45].
But the infection by the virus itself is not only discussed as a cause of SAT. e occurrence of this disease has also been detected after the injection of the new vaccines [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Recent studies show that the spike protein could be detected in subjects after administration of the mRNA vaccine [30] because both vaccines lead to endogenous production of these spike proteins through different mechanisms to induce immunity against SARS-CoV-2 [6,7].
In addition, animal experiments have shown that this protein alone can cause cell damage [29]. Since the thyroid cells, as already described, show a high level of expression of the ACE2 receptor, which serves as a cell receptor, direct damage by the protein could be an explanation. However, this theory should be explored in more detail in further studies, and of course, the idea cannot be applied to other vaccinations (e.g., influenza) and the occurrence of SAT.
Another explanation that should be mentioned is the "autoimmune/inflammatory syndrome by adjuvants" known as ASIA. Bragazzi et al. summarized 50 cases of SAT after vaccination that were attributed to this syndrome [28]. Adjuvants are substances that can increase the effectiveness of vaccines and, in some cases, can cause relevant damage by triggering immunological phenomena through activation of B lymphocytes or molecular mimicry [46]. In the case of the vaccine BNT162b2 (BioNTech/Pfizer), lipid nanoparticles (e.g., ALC-0159) have an adjuvant effect in terms of stabilizing the mRNA and transporting it to the target site [47,48]. In contrast, the vaccine ChAdOx1 (the University of Oxford and Astra-Zeneca) does not contain any relevant adjuvant substances in the true sense [49] but processed polysorbate 80 (E 433), which is part of AS03, an adjuvant of influenza vaccines [50]. So, ASIA could also be a possible cause of the development of SAT after vaccination against COVID-19. However, ASIA cannot explain the association of SAT with the infection itself.
Our cases are in line with the published cases of SAT after COVID-19 vaccination so far (Table 1). e typical clinical presentation with neck pain and malaise, as well as the ultrasound of the thyroid gland and the time since administration (1-3 days), confirm the diagnosis of vaccination-associated SAT. In contrast to most of the cases reported in the literature, our first patient had completely normal thyroid function due to a longer history since the onset of symptoms. Among the 26 cases, there are three others who were euthyroid, and only 62% of the patients had hyperthyroidism. e positive TgAb in our second case was interpreted as unspecific. Nishihara et al. 2019 examined 40 patients in the early phase of SAT and were able to show that 52.5% had positive TgAb, which decreased over time [37]. ree of the 26 cases of vaccine-associated SAT also had positive TgAb.
We must note that although we ruled out corona infection by antigen testing, we did not arrange for further virologic testing for other pathogens. However, there was no evidence of infection.
We would like to point out that, based on the accumulation of case reports, SAT appears to be a possible but rare sequelae of SARS-CoV-2 vaccination that should be considered by practicing physicians. e course of SAT is often mild and disproportionate to the harmfulness of COVID-19 but may be important because of the substantial increase in the global vaccination rate, with more than 9.5 billion doses of vaccine administered [4].
Nevertheless, temporal findings can be no proof of a causal relationship. To date, there have been many reports of SAT following SARS-CoV-2 infection. But a study from a region in northern Italy heavily affected by SARS-CoV-2 could not document a rise in cases of SAT in their outpatient emergency clinic during the pandemic [51]. Likewise, the temporal association of the reported cases of SAT after vaccination against SARS-CoV-2 could still be a coincidence.
Because case reports can only raise suspicion of a causal relationship, studies should compare the incidence of SAT in the weeks after vaccination with an unvaccinated group or with the incidence of SAT before vaccination. In addition, when SAT is diagnosed in temporal association with vaccine use, physicians should make a report to the appropriate pharmacovigilance institution.

Data Availability
e data used to support the findings of this study are included within the article.

Additional Points
Subacute thyroiditis (SAT) is a temporary inflammatory disease of the thyroid gland that is probably triggered by multiple viral infections with a genetic predisposition to HLA-B35. In the past, the occurrence of SAT has been described, for example, after influenza vaccination. Recently, several reports have described the appearance of SAT as being associated with the injection of the new COVID-19 vaccines. Cross-reactivity between the spike protein and tissue proteins of the thyroid gland, ASIA, or the circulating spike protein as a cause for SAT after vaccination should be discussed.

Consent
Written informed consent has been obtained from the patient (or patient's guardian) for publication of the submitted article and accompanying images.

Conflicts of Interest
e authors declare no conflicts of interest.

Authors' Contributions
e patients described were medically advised by Dr. Kauka, Goebel, and Prof. Kann, who gave full approval. e cases were presented by Dr. Adelmeyer.